1. Field of the Invention
The present invention relates to a fuel injection control apparatus for an internal combustion engine.
2. Description of the Related Art
Multi-cylinder internal combustion engines, such as automobile engines, in which a fuel injection valve that injects fuel (gasoline) from an intake passage to a combustion chamber is provided in each cylinder have been put to practical use. When a fuel-injected engine is started, a required fuel injection amount for starting up the engine is computed as a start-up injection amount command value. Then, after the crankshaft angle is determined through cranking for starting the engine, amount of fuel corresponding to the start-up injection amount command value is injected from the intake passage to the combustion chamber through drive control of the fuel injection valve for each cylinder.
The cylinders in the above multi-cylinder internal combustion engine are grouped into a forward-stroke cylinder group and a return-stroke cylinder group. The forward-stroke cylinder group is a group of cylinders in which the amount of fuel corresponding to the start-up injection amount command value may be injected by an early period of first intake valve opening after the crankshaft angle is determined. Meanwhile, the return-stroke cylinder group is a group of cylinders in which the amount of fuel corresponding to the start-up injection amount command value cannot be injected by the early period of first intake valve opening after the crankshaft angle is determined.
As a fuel injection mode for injecting the amount of fuel corresponding to the start-up injection amount command value after the crankshaft angle is determined, a fuel injection mode in which the fuel amount corresponding to a required value for starting the engine when the fuel in the combustion chamber is combusted may be employed. And such fuel injection modes include sequential injection and grouped injection that are described in Japanese Patent Application Publication No. 7-158482 (JP-A-7-158482) (paragraphs [0004], [0005], [0019], and [0030]), for example.
Sequential injection is a fuel injection mode in which the amount of fuel corresponding to the start-up injection amount command value is injected into each cylinder in an early period of the first intake valve opening in order to reduce the exhaust emissions of the internal combustion engine. The reason exhaust emissions may be reduced by injecting an amount of fuel corresponding to the start-up injection amount command value in the early period of the first intake valve opening is because the time is required for the fuel to enter the combustion chamber after being injected, and thus the vaporized fuel tends to enter the combustion chamber, which suppresses entry and combustion of liquid fuel in the combustion chamber. If liquid fuel is combusted in the combustion chamber, the amount of hydrocarbon (HC) and smoke (soot) in exhaust emissions are increased. Such increase in exhaust pollutants may be suppressed through the sequential injection.
The grouped injection is a fuel injection mode in which the cylinders at the same position of a piston in the forward-stroke cylinder group and the return-stroke cylinder group are grouped, and the amount of fuel corresponding to the start-up injection amount command value is injected at the same start timing for the same injection period in the cylinders of the same group. Considering the startability of the internal combustion engine, it is conceivable for the grouped injection that the injection of the amount of fuel corresponding to the start-up injection amount command value may be completed by the early period of the first intake valve opening in each cylinder in the forward-stroke cylinder group. If the injection of the fuel amount corresponding to the start-up injection amount command value continues after the early period of the first intake valve opening, the fuel will not be efficiently supplied. It is because the supply of fuel into the combustion chamber is interfered with either by a closing movement of the intake valve from a fully open state in a late period of the intake valve opening or by the closed intake valve. Because of the inefficient fuel supply as described above, the startability of the internal combustion engine may deteriorate. However, such deterioration in startability is prevented by the above setting of the fuel injection start timing in the grouped injection.
When mixed fuel of gasoline and alcohol is used as fuel for the internal combustion engine, and when the above sequential injection or the grouped injection is adopted as the fuel injection mode for the injection of fuel amount corresponding to the start-up injection amount command value after the determination of the crankshaft angle, the fuel with a high alcohol concentration may deteriorate the startability of the internal combustion engine. This is related to a decrease in volatility of the mixed fuel with an increase in an alcohol concentration in the fuel in comparison with gasoline fuel. In other words, if either the sequential injection or the grouped injection, which are the fuel injection modes set for use of gasoline fuel, is executed after the determination of the crankshaft angle without consideration of such volatility characteristics of the mixed fuel, the amount of fuel to be combusted in the combustion chamber after the determination of the crankshaft angle becomes insufficient. As a result, the fuel is not favorably combusted in the combustion chamber, and thus the startability of the internal combustion engine may deteriorate.